The present invention relates to control systems for dynamoelectric machines and, more particularly, to a method for determining the available output power of an alternator operating at a constant speed.
Dynamoelectric machines such as alternators are used in various applications to develop electric power. In an exemplary application such as a diesel-electric locomotive, an on-board alternator is driven by a diesel engine to provide electric power to traction motors coupled in driving relationship to wheels of the locomotive and to provide electric power for other electrical apparatus in the locomotive. Electric power for such other apparatus is commonly referred to as "hotel" power and may be used to power various electric appliances designed to operate on fixed frequency, typically 60 H.sub.z, alternating current (AC) power. Since the frequency of the electric power generated by the alternator is determined by the rotational speed of the alternator rotor which is driven by the diesel engine, it is desirable to operate the engine at a constant speed or RPM. In one conventional system, the engine operates at 900 RPM to produce 60 H.sub.z power from the alternator.
In locomotives, multiple alternators are coupled to an output drive shaft of the diesel engine with each alternator providing power for specific purposes. For example, one alternator may provide traction power to the traction motors, another alternator may provide power for charging batteries and still another alternator may provide the above-described hotel power. The electric power available from such alternators, given a constant RPM, is determined by the field current in field windings of the alternators. However, as additional electric power is demanded from the alternators, such demand is reflected as additional loading on the diesel engine. If the electric power demand is increased to a value which overloads the diesel engine, the engine may "bog" down. If the engine is able to maintain RPM under the overload condition, it may begin to "smoke" badly from incomplete combustion in the engine cylinders possibly damaging the engine. If the overload is such as to force a drop in engine RPM, the frequency of the hotel electric power will drop proportionately and may result in damage to the electric appliances receiving the hotel power.
In order to avoid the above disadvantages of overloading of the diesel engine/alternator power systems, such systems may include power monitors to measure the magnitude of power being drawn by such hotel loads. The diesel engine is specified to have capability of providing a maximum magnitude of electric power without overloading. Some portion of this electric power may be supplied as hotel power and the remainder supplied as propulsion power to the traction motors. In one exemplary system, the hotel power alternator may be capable of supplying up to 800 kilowatts (KW) of hotel power while the traction alternator can supply electric power to the traction motors for producing up to 2800 horsepower (HP). If the diesel engine has a maximum limit of 2800 HP, the power monitor is adapted to provide a control signal representative of the magnitude of hotel power so that the maximum available remaining power can be supplied to the traction motors, if desired, without overloading the diesel engine, thus allowing the locomotive to operate at a higher power or speed.
A disadvantage of the above described system is the requirement for a power monitor, e.g., a wattmeter, to determine the output power provided to the hotel loads. The wattmeter is relatively expensive, fragile and affected by the incessant vibration of the locomotive. Accordingly, it is desirable to provide a method for determining available traction power without use of a power monitor.